1. Field of the Invention
The present invention relates to a detection apparatus which detects an error component contained in two position signals approximated by a cosine function and a sine function representing a position of an object to be detected.
2. Description of the Related Art
A detection apparatus which detects error components contained in two position signals approximated by a cosine function and a sine function which represent a position or an angle is industrially used in the wide range of fields such as an encoder or an interferometer. These position signals are approximated by two expressions, for example, A=G cos θ and B=G sin θ.
These position signals are converted to digital signals using an A/D converter, and an arctangent calculation is performed using a micro processing unit (MPU) or a digital signal processor (DSP). According to these calculations, a value (phase θ) proportional to the position or the angle can be obtained with high resolution.
An offset error, an amplitude error, or the like is contained in the signal outputted from the detector. As a detection apparatus which corrects these errors to perform detection, for example, U.S. Pat. No. 4,458,322 has been known. Furthermore, as another technology for correcting an error, for example, U.S. Pat. No. 5,581,488 has been proposed. U.S. Pat. No. 5,581,488 discloses a method for correcting an error caused by a phase difference error, a second order distortion, and a third order distortion, in addition to the offset error or the amplitude error.
The position signal outputted from the detector contains an error component constituted by a various kind of frequency components, which are different from the ideal cosine function and sine function. For example, the signal which is obtained by dividing two position signals outputted from a head of the detector by an amplitude G is represented by expressions 1 and 2 below. The coefficients will be explained later.A/G=cos θ+ZA+g cos θ+h sin θ+ . . . +pk cos kθ+qk sin kθ  (1)B/G=sin θ+ZB−g sin θ+h cos θ+ . . . +rk cos kθ+sk sin kθ  (2)
As a result, it is recognized that the position signal (with phase θ) that is an obtained position or angle contains a various kind of error components. The conventional detection apparatus (error correction technology) has performed correction independently for each error factor. Therefore, there has been a problem that the detection apparatus is extremely complicated in order to deal with a lot of error factors.